kernel.bbclass: consider .csp firmware files

kernel.bbclass: consider .csp firmware files

Fixes kernels which package sb16 firmware e.g.

directories were installed but not shipped in any package:
  /lib/firmware/sb16
  /lib/firmware/sb16/ima_adpcm_playback.csp
  /lib/firmware/sb16/ima_adpcm_init.csp
  /lib/firmware/sb16/mulaw_main.csp
  /lib/firmware/sb16/ima_adpcm_capture.csp
  /lib/firmware/sb16/alaw_main.csp

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>